1. Field of the Invention
The present invention relates generally to cancellation of intra-channel and inter-channel interference in coded spread spectrum wireless communications systems having multiple transmit antennas and employing closed loop channel information fed back from a receiver to a transmitter. More specifically, the invention exploits spatial diversity afforded by multiple transmit and receive antennas, in combination with an interference-cancellation unit that performs symbol-estimate weighting, subtractive cancellation with a stabilizing step-size, and mixed-decision symbol estimation.
2. Discussion of the Related Art
In an exemplary wireless multiple-access system, a communication resource is divided into code-space subchannels allocated to different users. A plurality of subchannel signals received by a wireless terminal (e.g., a subscriber unit or a base station) may correspond to different users and/or different subchannels allocated to a particular user.
If a single transmitter broadcasts different messages to different receivers, such as a base station in a wireless communication system serving a plurality of mobile terminals, the channel resource is subdivided in order to distinguish between messages intended for each mobile. Thus, each mobile terminal, by knowing its allocated subchannel(s), may decode messages intended for it from the superposition of received signals. Similarly, a base station typically separates signals it receives into subchannels in order to differentiate between users.
In a multipath environment, received signals are superpositions of time-delayed and complex-scaled versions of the transmitted signals. Multipath can cause several types of interference. Intra-channel interference occurs when the multipath time-spreading causes subchannels to leak into other subchannels. For example, forward-link subchannels that are orthogonal at the transmitter may not be orthogonal at the receiver. When multiple base stations (or sectors or cells) are active, inter-channel interference may result from unwanted signals received from other base stations. These types of interference can degrade communications by causing a receiver to incorrectly decode received transmissions, thus increasing a receiver's error floor. Interference may degrade communications in other ways. For example, interference may diminish the capacity of a communication system, decrease the region of coverage, and/or decrease maximum data rates. For these reasons, a reduction in interference can improve reception of selected signals while addressing the aforementioned limitations due to interference.
The use of multiple transmit antennas provides flexibility for shaping transmissions and, if intelligently used, allows for interference cancellation to improve receiver performance over what can be accomplished with a single transmit antenna.
Multiple transmit antennas may be employed in a wireless system with the aim of increasing the signal quality at the receiver. This improvement may be obtained by beamforming, in which case the channel linking each transmit and receive antenna pair is measured at the receiver and fed back through a dedicated channel to the transmitter. The transmitter may determine a set of complex weighting coefficients (each corresponding to one antenna) that adapt the phase and/or amplitude of a common information signal transmitted to the receiver across each antenna. The weights may be chosen to maximize power at the receiver, subject to a constraint on the total transmit power. Alternatively, the receiver may calculate the weights, which may be fed back across the dedicated back channel. In this manner, an additional diversity advantage may be gained over the wireless medium and the (average) inter-channel interference may be reduced, as the base stations do not transmit power isotropically to all users in their respective cells.